1. Field of the Invention
The invention relates to a device for the spatial ultrasonic location of concrements.
2. Related Art
So-called lithotriptors are used to remove concrements, for example kidney stones or gallstones, found in the body of a living thing. With these lithotriptors, such concrements can be destroyed, without being touched, by means of focused ultrasonic shock waves. By applying ultrasonic shock waves, surgery or the introduction of probes into the body of a patient can be avoided. Thus, the danger of infection associated with such interventions is also avoided.
To prevent damaging healthy tissue in the area surrounding the concrement, the concrement to be destroyed must be positioned as exactly as possible in the focus of the shock wave. X-rays or ultrasonics, for example, can be used to locate the concrement spatially, as is required for the exact positioning.
An important advantage in an ultrasonic location of the concrement to be destroyed is that, simultaneously with the location procedure, information can be gained concerning the conditions for the sound propagation of the shock wave. This information is obtained when the area in the body of the living thing, which is irradiated by the ultrasonic shock wave, is also covered by the ultrasonic waves used for the location procedure.
For example, devices for the spatial ultrasonic location of concrements are known from German Patent No. 27 22 252. They receive the reflections and measure the propagation delay of pulses, which are emitted by an ultrasonic transmitter and reflect off the concrement or scatter, to locate the concrement. In one specific embodiment, an ultrasonic transmitter and an ultrasonic receiver are mounted in the wall of a coupling device. Their axes of rotation intersect at an angle of 30.degree. in the focus of the ultrasonic shock wave. This coupling device contains the shock wave source and can be placed externally on a patient. Instead of using a separate ultrasonic transmitter, an arrangement is also provided, whereby the shock-wave source, operated with reduced intensity, is itself used as an ultrasonic transmitter to locate the concrement. By means of pressure sensors, which are arranged in the wall of the coupling device, the scattered shocks emanating from the concrement are received. The position of the concrement is then determined from these differences in propagation delay. In a further specific embodiment, a swivelling ultrasonic transformer is provided. In accordance with the B-scan process, it generates a sectional view in a plane containing the focus of the ultrasonic shock wave.
As far as a reliable and simple positioning of the concrement in the focus of the ultrasonic shock wave is concerned, of the known ultrasonic detection devices, the device working according to the B-scan process is more advantageous than devices working according to the A-scan process. This is due to the fact that, on the one hand, the two-dimensional B-scan gives a clear impression of the geometric proportions of the area surrounding the concrement and, on the other hand, one can interpret it more reliably than a one-dimensional A-scan.
However, echographic B-scan processes have the disadvantage that only sectional planes of a body can be depicted, which run perpendicular to the axis of the body and thus, for the most part, parallel to the direction of propagation of the ultrasonic waves. Thus, the image information, which is available to the user for positioning the focus or the concrement, is of an unfamiliar object plane, which is ill-suited for sharp focusing. In the case of the known device, for example, when a concrement is displaced lateral to the image plane in the image relative to the focus, this corresponds to a defocusing. This displacement of the concerement can consist of a lateral, as well as of an axial displacement, relative to the direction of propagation of the ultrasonic shock wave.
A further disadvantage of the know device in this respect is also that, in particular, the lateral resolution of an echographic sectional view, which is conditional on the width of the sound cone used for scanning, is unsatisfactory and hinders an exact axial location and positioning of the concrement in the focus of the ultrasonic shock wave.
An ultrasonic imaging system for diagnostic purposes, which operates in accordance with the principle of an optical episcope, is known from "Acoustical Holography", vol. 5, Plenum Publ. Corp., New York 1973, Ed. P. S. Green, pages 493 to 503. This system, called an ultrasonic camera, allows sectional views of the patient to be displayed, which run perpendicularly to the sagittal sectional planes of the ultrasonic echography. For this purpose, the patient to be examined is "illuminated" with ultrasound. Either the transmitted or reflected ultrasonic waves, scattered by the patient, are focused by means of a lens system onto an image plane and converted into electric signals by a linear receiving array arranged there. With the help of two contra-rotating prisms arranged in the path of rays, the image generated by the lens system is deflected sinusoidally, so that a two-dimensional image can be constructed with the receiving signal measured on the linear receiving array. The image frequency attained in this way amounts to approximately 15 Hz, so that one can already speak of a real-time image presentation. A higher image frequency, however, is not possible. This is due to the fact, that the higher rotational frequency of the prisms, which this would require, would cause turbulence in the sound-carrying liquid and thus lead to interference in the sound propagation and a reduction of the image quality. The depth level of the sectional plane focused by the ultrasonic camera results, according to the principles of geometric optics, from the characteristics of representation of the acoustical imaging system being used, the image distance of the receiving array, and the position of the body relative to the then established focal area in the object field.
An ultrasonic transmission camera, which enables a higher image frequency without mechanical moving parts with a simplified design of the scanning device, is revealed, for example, in "Acoustical Imaging", vol 15, Plenum Publ. Corp., New York 1987, Ed. H. W. Jones, pages 213 to 225. Therein, instead of a linear receiving array, a two-dimensional receiver matrix consisting of 29.times.128 transducer elements is provided. The electric signals impinging on the individual transducer elements are read out one after another and merged into a two-dimensional image, whose image frequency amounts to approximately 25 Hz. The area of the receiver matrix which is sensitive to ultrasound is formed, thereby, of a thin PVDF foil, which is pressed against a matrix-shaped electrode arrangement. A further development of this type of receiver matrix is known, for example, from the U.S. Pat. No. 4,742,494.